The present invention relates generally to the field of optodes. Optodes are optical sensor devices that optically measure the concentration of a specific substance within a fluid (a liquid or a gas), usually with the aid of a chemical transducer. More particularly, the invention is directed to a method, and to an improved optode that implements the method, whereby the optode is capable of conducting a self-test of its measurement quality, to determine whether the optode is no longer providing accurate measurements. The method and the improved optode are discussed primarily in connection with the measurement of dissolved oxygen in water, but it should be understood that they could also be used to measure other chemicals in other types of fluids.
Accurate measurement of the concentration of dissolved oxygen in a body of water is important for a variety of reasons. For example, low oxygen concentrations are a common water quality problem downstream of hydropower facilities. Many hydropower operators measure oxygen concentrations to insure that they are within regulatory compliance.
A luminescence-based optode typically includes a sensor cap that is coated with a luminescent material within a polymer matrix (the chemical transducer), circuitry to illuminate this material, and circuitry to detect the material's luminescent emission. The optode is designed such that blue light (the excitation light) from a light source strikes the luminescent material on the optode cap. Photons of this blue light collide with electrons of the luminescent material, providing the electrons with sufficient energy to boost (excite) them into higher energy orbits. These electrons cannot remain in the excited state indefinitely and, over time, return to the initial, unexcited state, losing the excitation energy. For the purposes of this invention there are two ways this energy is lost. The first is that the energy is emitted as photons of red light. The second is that the energy is lost, without emission of photos, by collision with a nearby quenching molecule. Both types of energy loss can and do occur at the same time. Only a fraction of an excited population of electrons will be quenched, leaving the remaining electrons to return to the unexcited state by emitting red light. Thus, the properties of the emitted light, such as its intensity and the lifetime of the emission, will change in a way that depends upon the concentration of quenching molecules. A photo detector and electrical circuits are used to measure characteristics of the emitted light. Optical filters are used to prevent blue excitation light from reaching this photo detector, so emission light can be measured at the same time as the material is receiving excitation light.
The phenomenon of quenching forms the basis for two common methods for determining the concentration of the “quenching” molecules, such as oxygen. The first method uses intensity-based measurements wherein the intensity of emission is measured and declines as the concentration of the quencher increases. The second method uses lifetime-based measurements wherein the characteristic time of decay of emission is measured and becomes shorter as the concentration of the quencher increases. Until the present invention, optodes have generally used only one of these two methods.
Optodes that use the intensity-based method operate by measuring the ratio of the intensity of excitation to that of the intensity of emission. Quencher molecules (e.g., oxygen) deactivate a fraction of the excited electrons and so reduce the number of electrons that emit and therefore reduce the emission intensity relative to the excitation intensity.
The optode is calibrated by exposing the luminescent material to different temperatures and concentrations of the quencher and measuring the resulting intensity ratio. An interpolation function fi is determined from the measurements to provide a mathematical expression that describes the functional relationship between the concentration of the quenching molecule, the intensity of the emission, the intensity of the excitation, and the temperature of the luminescent material, as follows:[C]=fi(Lem/Lex,T)
where                [C] is the concentration of the quencher,        Lem is the intensity of emission,        Lex is the intensity of excitation, and        T is the temperature of the material.        
This intensity-based method is relatively simple to implement but it is known to suffer from defects. Often, for cost and convenience reasons, only Lem is monitored, and Lex is assumed constant. Common sources of Lex, such as light emitting diodes (LEDs), gradually loose intensity over time. In that case, variations in Lex will be mistakenly interpreted as variations in Lem, and the measurement of [C] will be inaccurate. Also, in practice, fi does not completely describe the response of the luminescent material. For example, fi is known to change as a function of the amount and duration of the excitation light, an effect that is known as “photo bleaching” of the luminescent material. The resulting changes in the true fi relative to the fi that was determined by calibrating the optode, result in inaccurate measurement of [C]. Finally, if variations in the optical path occur that change the intensity of excitation or emission light individually, then Lem/Lex will change and the measurement of [C] will be inaccurate. Such variations in the optical path can be caused by condensation of water within the optics or movement of the optics due to external stresses or rough handling of the optode.
Optodes that use the lifetime-based method operate on the principle that in the quenched luminescent material an impulse excitation creates a population of excited electrons that leave the excited state at an exponentially decaying rate, with the exponent described as the ‘characteristic time’. Quencher molecules (e.g., oxygen) deactivate a fraction of the excited electrons and so reduce the number of electrons that emit and, therefore, reduce the characteristic time of the emission. It can also be shown that a system of this type will exhibit a response to the application of an excitation light having sinusoidally varying intensity, by producing an emission light having a sinusoidally varying intensity at the same frequency as the excitation light, but delayed by a specific amount of time, a ‘phase shift’. The measurement of phase shift is a common method used within optodes for determining the characteristic time.
The optode is calibrated by exposing the luminescent material to different temperatures and concentrations of the quencher and measuring the resulting characteristic time. An interpolation function ft is determined from the measurements to provide a mathematical expression that describes the functional relationship between the concentration of the quenching molecule, the characteristic time (or phase angle) of emission, and the temperature of the luminescent material, as follows:[C]=ft(t,T)
where                [C] is the concentration of the quencher,        t is the characteristic time of emission, and        T is the temperature of the material.        
This is a more complex measurement to make, and requires more complex electronics, but the lifetime-based measurement method does not suffer dramatically from the problems of the intensity-based measurement method described above. There is one subtle problem, however. In practice, the measurement of the characteristic time becomes more difficult as the emission intensity declines (for any of the reasons discussed in the intensity-based method above). In general, the measurement of characteristic time exhibits good stability over ranges of high intensity with problems occurring at low intensity. The lifetime-based method, although better than intensity-based, is not completely insensitive to intensity. Despite this subtle problem, many commercial sources of oxygen optodes use this lifetime-based method to evaluate the quenching effect and to determine the oxygen concentration.
Regardless of which method an optode implements—the intensity-based method or the lifetime-based method—the quality of the measurements made by the optode optode will eventually degrade. It is important that an optode not be used to collect data after its measurement quality has degraded below an acceptable level because incorrect measurements can lead to incorrect decisions. Continuing the example above, an incorrect measurement of oxygen could lead a hydropower operator to believe that they are not compliant with regulations and cause them unnecessary expense.
Normally, one cannot know whether an optode is making quality measurements without removing the optode from service and testing it. It would be beneficial and advantageous to have a way to determine when the quality of an optode's measurements has degraded, without having to remove the optode from service to test the optode.